Every time it rains on Worthington, it means trouble for Okabena Lake.
When it rains, soil and nutrients wash down from the town and from thousands of acres of farmland nearby. The impact on the lake is immediate and startling.
"We can have the sediment coming into the lake turning the lake a brown color, and not have it clear up for the rest of the season after one big rain event," said Dan Livdahl, who heads the local watershed district. "Our water clarity will go from pretty good to terrible."
It's a frustrating transformation — one Livdahl and his colleagues around Minnesota witness each summer. The green scum that spreads across thousands of Minnesota's contaminated lakes is the legacy of decades of pollution from cities and farms.
Grow the Future of Public Media
MPR News is supported by Members. Gifts from individuals power everything you find here. Make a gift of any amount today to become a Member!
Those lakes can be rescued, but it's a massive undertaking that requires farmers, firms and homeowners to change the way they do business. Cooperation and compromise helped save Lake Shaokatan, 90 miles north of Worthington.
At Okabena, though, it's still a work in progress. The lake is a recreational hub that's become a hotspot for national windsurfing competitions. Those with a stake in Okabena agree the lake needs help and there have been small measures of improvement, but no breakthroughs.
Homeowners around the lake note the water gets more cloudy in late summer. Months of warm temperatures can give rise to algae blooms that produce what one researcher described as an "overwhelming sewage-like smell."
"When the wind is in the wrong direction, depending upon where you're standing, you'll certainly know it's there," Lake Okabena Improvement Association President Jason Vote said as he sat on his lakeshore dock. "It'll stop walkers and runners around the lake because it gets really, really bad."
Houses and parkland line Okabena's 1.5-mile shoreline. The average depth of the brown-green water is about 7 feet.
When Livdahl recently checked the water's clarity using a Secchi disk, the 8-inch black and white plate disappeared into the murk after about 2 feet. Not bad for Okabena, but in a clear Minnesota lake, the disk might still be visible at 10 or 20 feet, or all the way to the bottom.
Okabena and many of Minnesota's other 4,000 shallow lakes haven't always been cloudy, algae-ridden bodies of water. Detailed analysis of core samples from the bottoms of several state lakes show that 150 years ago or so, they were fairly clear.
Changes came after farmers broke the prairie in the late 1800s and urban areas began developing. Subsequent runoff carried high levels of phosphorus and nitrogen into the lakes, creating a crippling cycle, said University of St. Thomas biologist Kyle Zimmer.
"The more of those two nutrients in a lake, the faster the algae can grow and the more the clear water state is destabilized," he said.
In a clear water state, the lakes had plentiful vegetation growing on the bottom, so for a time, those plants soaked up the extra nutrients pouring in. They eventually lost out, Zimmer added.
"If nutrient levels get so high that the growth rates of submerged aquatic plants can't keep up, that's when the system gets basically overloaded and all the nutrients start shifting over to algae up in the water column and you flip over to a turbid water state," said Zimmer.
The flourishing algae cut off sunlight to submerged plants, killing them. That freed up even more nutrients for the algae. Eventually, they clouded the water and at times matted the lake surface.
To make matters worse, the fish populations changed to more carp and other rough fish that thrive in poor quality waters. These bottom feeders stir up sediment releasing soil and trapped nutrients, further clouding the waters.
Okabena has seen all of this happen, and there are efforts now to boost the lake's water quality. But there's no complete agreement, for instance, on how far farmers must go to slash the runoff from their land.
Farmland runoff is a major problem. The Minnesota Pollution Control Agency estimates rural drainage is responsible for most of the phosphorus entering Okabena.
Bill Gordon and his family farm a couple of miles from the lake and have taken several steps on their land to improve the water quality.
Gordon said his family has converted roughly 10 percent of their 2,000 acres to protective cover, mainly grass and wetlands. These natural filters remove soil and nutrients from runoff before it enters the lake.
The family also signed easements turning 35 acres of their land into water holding areas. The ponds allow pollution to settle out before it's released through a spillway to the lake.
"Okabena's a great asset to Worthington," Gordon said. "It's one of our attractions, it's great for recreation."
Despite help from Gordon and other farmers in the area, however, state studies suggest more needs to be done. The MPCA estimates farmers need to slash their share of Okabena Lake pollution another two-thirds. Gordon and other farmers are balking.
Gordon said he disagrees with the MPCA findings and believes they overestimate cropland runoff. Farmers, he added, could not give up enough cropland or fertilizer use to meet that goal and survive financially.
"We want healthy, safe, water. That's the most important thing," said Gordon. "But use common-sense, cost-effective answers. We can get this done."
State water managers have demonstrated they can restore an algae-ridden, turbid lake. But it can take years.
Lake Shaokatan near the South Dakota border is largely seen as a southwest Minnesota success story. But the restoration took more than two decades.
Twenty-five years ago, the lake was often covered with algae blooms in the summer. But residents banded together to come up with a plan to improve water quality.
"The lake had the inherent capability to clean itself up if we could help it and try and control some of things that were going on — that maybe we're bringing nutrients to the lake," said Bill Doyscher, a Lake Shaokatan homeowner.
With the help of state and federal agencies and area farmers, nutrients were sharply reduced.
Some nearby livestock feedlots took steps to prevent manure from reaching the lake. Shaokatan residents fixed their septic systems to keep seepage from the water. Wetlands were rehabilitated to hold and clean runoff before it entered the lake.
The result was an almost tenfold drop in phosphorus peak levels, from about 370 parts per billion to about 40 ppb in 2014. The water cleared and vegetation reestablished itself on the lake's bottom.
"I think it's in a lot better shape than it was when we first came out here," said Doyscher as he walked the length of his dock. "The water quality is better."
In the complex world of water management, however, every advancement comes with a price.
The new vegetation exploded as it soaked up Shaokatan's phosphorus and other nutrients. It broke the surface, leaving mats of vegetative barriers for boaters.
"By the end of the summer it was pretty well covered," said Doyscher. "Travel one end of the lake to the other and sometimes it didn't work."
Scientists are looking now at methods to control the vegetation. Even with this new problem, most lake residents would rather have the clearer water then go back to algae blooms, Doyscher added.
That's a choice Worthington's 13,000 residents may also face with Okabena, which remains heavily polluted with hardly any vegetation growing on the lake bottom to absorb nutrients from the water. Saving it will take a lot of work.
Resident Terry Morrison, though, said he believes cleaning up the lake is an obligation and that how well the job is done will serve as a measuring stick of the region's moral and ethical standing.
"Water to me really means life," said Morrison. "I believe that it represents the overall health of our existence. Without water,we simply die."